Embodiments of the invention relate, in general, to electricity generating installations and, specifically, to gas turbine or combined-cycle power plants in which a gas turbine and a steam turbine are used in combination to drive a generator for the production of electricity.
Existing electricity generating installations generally involve the use of large-scale hydroelectric, nuclear or fossil fuel-fired power plants which supply electricity via transmission and distribution systems.
Targets for the reduction of CO2 emissions set by numerous countries will lead to an increase in the proportion of power generation from renewables which, for example, will reach 35% in Germany and 20% in France by 2020. In addition to improvements in efficiency, conventional electricity supply facilities will be required to show greater flexibility and responsiveness in the interests of their optimized operation, and will need to accommodate a wider variety of intermittent power sources, such as wind turbines, solar power plants and other facilities, such as wave-powered, geothermal or biomass plants. This diversification, and the associated increase in the number of production sources, will pose a considerable challenge to network management and electricity distribution systems.
Existing electricity production facilities and distribution networks were not designed to accommodate these changes and, as a result, are ill-suited to the fulfillment of these new requirements in the long term, in the absence of substantial investments for this purpose.
Electricity production from renewables at this level increases the complexity of electricity distribution systems and networks, resulting in fluctuations in energy supply conditions which will need to be carefully managed. In the absence of advanced control, there is a risk that distribution systems will operate inefficiently, or will be subject to frequent disturbances.
From the viewpoint of electricity suppliers and distribution system operators, potential solutions are as follows: the increased flexibility of conventional power plants; the introduction of energy storage technologies designed for use at all levels, as a means of offsetting peaks in demand and allowing the incorporation of a greater number of renewable energy sources; the introduction of more flexible distribution methods as a means of accommodating fluctuations in supply, improving efficiency and optimizing system operation; and the introduction of high-efficiency forecasting, monitoring and control systems, as a means of countering any disturbances.
Disturbances which are liable to ensue from the planned modification of electricity sources and distribution networks may result in power outages (power cuts), i.e. the short-term or long-term loss of electricity within a given zone, associated with faults on power plants, damage to the distribution system (electricity transmission lines or substations), a short-circuit or overload on the electricity network.
Specifically, a “blackout”, or network outage, is a particularly critical issue for public safety, hospitals, sewage treatment plants, mines, etc. Other critical systems, such as telecommunication systems, must also be provided with an emergency power source. For this reason, installations are provided with standby generators, which will start up automatically in case of an interruption in electricity supply.
The occurrence of faults on an electricity network in proximity to a power plant of the gas turbine, steam turbine or combined-cycle type may also generate disturbances, or may even result in the shutdown of the power plant concerned.
Moreover, an electricity generating plant taps electricity from the network in order to initiate the run-up of turbine speed, using a generator operating in motor mode, and for the supply of power to the auxiliary systems of the power plant. These power plants must also be provided with standby power supply facilities, such as batteries or diesel engines, in order to accommodate micro-outages of several seconds' duration, or to ensure the completion of normal shutdown and, where applicable, restarting in case of the loss of the network.
Energy storage facilities distributed throughout the network may be used for the regulation of frequency variations, the rapid adjustment of electricity supply to meet demand, the accommodation of highly fluctuating levels of production from power plants using renewable energy sources, and the supply of standby electricity following a power outage.
The function of frequency control is also intended to reduce frequency deviations on networks. Frequency deviations result from imbalances between electricity supply and demand, which may occur at any time during normal operation of the system, or further to an incident such as a loss of production. In Europe, the nominal frequency is set at 50.00 Hz. The minimum instantaneous frequency is set at 49.2 Hz and the maximum instantaneous frequency is set at 50.8 Hz. This corresponds to a frequency deviation of 800 mHz, the maximum permissible dynamic deviation in the nominal frequency (ENTSO-E 2009). In practice, instantaneous frequency ranges are larger, ranging from 46 Hz to 52.5 Hz.
There are three levels of frequency control, namely, primary control, secondary control and tertiary control.
Under rated operating conditions, power plants are required to maintain a reserve capacity for the purposes of a primary frequency control response. In Europe, this reserve capacity may vary from country to country. For example, this reserve capacity is +/−2.5% in France and +/−1.5% in Spain.
The deployment of the primary reserve capacity is initiated before the deviation from the nominal frequency exceeds 200 mHz, within a time interval of 30 seconds and for a maximum duration of 15 minutes.
Accordingly, energy storage means may also be used as a means of contributing to frequency control, in continuous duty and with a rapid response capability.
Finally, it is necessary to regulate any voltage-current phase difference by means of reactive power control. System loads which incorporate windings have a magnetizing effect, resulting in the generation of reactive power. Although the latter delivers no work, in vectorial combination with the active power (chargeable capacity), it constitutes the apparent power which defines the total energy circulating on the network, and also dictates the dimensioning of installations. By optimizing the power factor, it is possible to reduce network losses, maximize the active power flow (or reduce the dimensioning of installations) and enhance stability. Here again, energy storage means may be used for the regulation of this phase difference.
Storage means may also provide the energy sources required for the start-up of power plants (absorption of peaks in capacity, etc.), preventing any micro-outages which are prejudicial to the continuous supply required by hospitals, data processing centers and the standby systems of nuclear power plants.
Kinetic energy or flywheel storage means are applied in this context. These systems, which are comparable to a mechanical battery, involve the rotation of a flywheel (of carbon fiber, other composite materials, steel, etc.) up to a speed of several tens of thousands of r.p.m., connected to a motor/generator. These systems can store/release any surplus/deficit of electricity on the network at any given time, in the form of kinetic energy (Ek) which is recovered by the acceleration/deceleration of the flywheel mass. The energy stored/released is given by the following formula:
            E      k        =                  1        2            ·      j      ·              ω        2              ,where l is the moment of inertia (in kg·m2) and ω is the angular speed (in rad.s−1).
In order to prevent friction losses, these storage systems are supported by magnetic bearings and are enclosed in vacuum housings. They are also provided with power electronics, such as a rectifier-inverter combination, for the purposes of the control of the signal injected/extracted into/from the network and, specifically, for the control of the power factor (cos φ).
This technology is used, amongst other applications, for frequency control, as a solution for the provision of an uninterruptible power supply, for the optimization of energy supply in on-board systems, and in fields such as electricity distribution, aerospace, motor vehicles (for the recovery of kinetic energy from braking), the rail industry, etc.
In the context of electricity distribution and the stability of electricity networks, these storage systems are highly advantageous, as they have a response time of less than one second, a service life of some twenty years, and require little maintenance. Moreover, unlike batteries, they have no “memory effect”, are not susceptible to variations in temperature, and permit the precise evaluation of their state of charge. Finally, they do not involve any recycling, and require no particular operating precautions.
At present, by way of reference, some devices of this type currently on the market have a mechanical efficiency in excess of 95% and an overall efficiency (for a complete charging/discharging cycle) of 85%. Some devices can achieve a storage capacity of 25 kWh, deliver an instantaneous capacity of 250 kW and undergo over 150,000 complete charging/discharging cycles.
Systems of various types for the prevention of interruptions in electricity supply, and for the control of frequency and power, are known from the prior art.
In this regard, reference may be made to documents EP 1 900 074 and EP 1 866 717 which describe supply systems of various types for the accommodation of peaks in consumption and the prevention of interruptions in service.
Specifically, document EP 1 866 717 recommends the use of a mini-network, comprising one or more electricity production sources and one or more independent system loads, which may be connected to the network in response to a disturbance.
The documents US 2005-0035744, EP 1 656 722, EP 359 027, U.S. Pat. No. 5,256,907, WO 2002-44555, U.S. Pat. No. 4,001,666 and JP 2003-274562 describe the use of flywheels.
Reference may also be made to document US 2004-0263116, which describes an intelligent energy distribution/storage system for demand-side capacity management. A device is used for the storage of energy in proximity to the point of use or point of production. Document JP 2003-339118 describes a distributed energy supply system comprised of a wind turbine, a photovoltaic generating unit, an energy storage unit, a flywheel and a charging unit.
In consideration of the above, embodiments of the present invention propose a control process for an electricity generating plant of the gas turbine or combined-cycle type, which counters the above mentioned disadvantages.
A gas turbine/steam turbine or combined-cycle power plant generates electricity directly. During start-up, however, the power plant is dependent upon electricity from the network for the supply of power to the generator in motor mode (in the case of a gas turbine), and for the supply of power to auxiliary systems required to power lubrication systems, fuel supply systems, cooling systems, heating and condenser blow-down systems, many of which are comprised of motor-driven pump units and motor-driven fans, valves, etc.
These facilities must be provided with a redundant power supply, in case of the loss of the network or a fault on the latter.
As indicated above, a number of electricity storage means are currently available which will ensure the operation or shutdown of electricity generating plants, such as gas turbine/steam turbine power plants and combined-cycle power plants. These means can be used for the purposes of load transfer, for the supply power to of pumps, fans or cabinets in case of a network or power plant fault, and for the delivery of a low and medium voltage supply for batteries.
In case of a network outage, or “blackout”, standby generating means must be provided which will allow h.v. and m.v. auxiliary systems to complete the shutdown of the shaft line and/or the start-up of the generator in safety mode, where applicable.
Typically, these means are comprised of diesel engines, some of which are redundant, which must be maintained in a pre-heated and pre-lubricated condition at all times, in order to be ready for start-up. Moreover, as a safety measure, these engines must undergo regular start-up tests.
Investments for the acquisition, installation, supply and conditioning of these storage and production facilities are considerable, given that they may be subject to only sporadic use.
Moreover, the progressive deterioration of batteries over time may generally be assumed, given that this deterioration is associated with each charging and discharging cycle (hysteresis).
A further problem is the response time of these storage and production means, and limiting this response time would restrict the impact of the fault concerned upon the network and the power plant.